Sub-zero Celsius treatment: A promising option for future martensitic stainless steels

A series of samples of (in wt.%) 11.5Cr-0.67C martensitic stainless steel grade were austenitized in Argon for 1 hour at temperatures ranging from 1010°C to 1190°C. Additionally, a series of samples of (in wt.%) 15.0Cr-5.8Ni-1.0Mo-0.03C (EN 1.4418) martensitic stainless steel grade were solution nitrided at 1110°C for 2 hours in N2 at a pressure of 1 atm. Some of the samples were subjected to a sub-zero Celsius treatment. Four types of sub-zero Celsius treatments were investigated: (i) isothermal holding at -78°C; (ii) immersion in boiling nitrogen followed by immersion in water; (iii) immersion in boiling nitrogen followed by isothermal holding at -78°C; (iv) isothermal holding at -78°C followed by immersion in boiling nitrogen and subsequently in water. The effectiveness of the different sub-zero Celsius treatments in promoting an additional hardening response was investigated by reflected-light microscopy and Vickers micro-hardness indentation. Complementary electron back-scatter diffraction was applied for determining the phase fractions of austenite and martensite. Data shows that sub-zero Celsius treatment yields an additional hardening response when austenite is retained in the material. The relevance of the additional response was observed to be a function of the sub-zero Celsius treatment applied, as well as of the stability of the retained austenite.